I was reading someone's opinion that fin length could be reduced by 1/3 with two fins compared to one and still get the same lift.
Would anyone care to comment on this?
I need extra clearance from the reef so every inch counts.

It sounds about right. But I'd consider a good weed fin first. Great weedies like those from Maui Ultra Fins or the speed weed fins from Black Project also let you sail a fin much shorter than a pointer. If you're already on another weed fin, you may find that an upgrade to one of those lets you go shorter, too.

I love the MUF slalom weed and speed weed for my freestyle and FWS boards - they are the only high performance fins I found for power box. They blow other weed fins (which I considered very good for years) out of the water. There's a test in a German windsurf magazine that confirms that.

For slalom boards, I like the Black Project WEEDSpeeds. Their "sizes" are not actual depth, but rather the equivalent size of a pointer fin; actual size is about 20% shorter. I was out on the BPF WeedSpeed 34 (actual depth: 28.2) today, and it felt absolutely bomb proof on a 90 l slalom board with a 7.0 sail in 18-30 mph winds and chop.

If you sail very flat water, or you are very good about keeping constant fin pressure, you should also consider the MUF Delta, which allows you to go much shorter. However, this is really a fin for flat & shallow water, where it works really well. In chop (even moderate Hatteras chop), it requires technique adjustment and considerable skill to keep it from spinning out. It does not give you the same lift as other fins (hence the technique adjustment). I'd get it a few cm longer than the MUF charts show. When it works, it's brilliant. Some better surfers than I can sail it in Kanaha.

On my Madd 135 twinwser I have 4 sets of weed fins I use, 11.5", 10.5" 26 cm(10.2") and 24 (9.4") cm. I use the 11.5 fins when the wind is 15 mph and with my 6.6 hucker can pump up on plane with the 8.5's, and use the 10.5 in 17-20 mph winds. I sail alot in the shallow waters of Hatteras and really enjoy the twinser boards. I have a RRD 115 twinser also.
If I had to choose one set of fins I quess it would be 10.5", but the 11.5" pump better.

This is my board, http://2002.star-board.com/products/freeformula198.asp
The problems are lack of appropriate wind, shallow water, my 220 lbs and lack of skills.
I can get it going with an 8M sail but the fin used is 52CM or about 20.5", this combo seems to work but I can only sail that fin in a smaller space where I won't hit the reef.
I had a crazy idea to have two A box's installed on the board and use much smaller fins which would double the length of my reach.

I have 2 MUF Delta Wings, one 19 cm and one 21. The biggest size they make is the 24 cm. That would definitely be too small for the Free Formula.

You could try the Slalom-Weed instead. I'm using the 36 cm with an 8.5 m sail. The 36 cm has about the same area as a 54 cm pointer fin. My board is narrower than yours, though (71 cm/117 l slalom). A 40-42 cm might work for your wider board with an 8M sail.

The reason I say an A box is because that's the box they put in twin fin boards so with the smaller fins it must be strong enough?
And it would be cheaper and easier and the local ding guy can do it no problem, not so sure about a Tuttle box, I know he's never done that before.
I figured I could just use some surfboard fins which are cheap and available.
A weed slalom fin would be the ticket, how do they size the weed fins? Is it the total length or the length it hangs down?

Whatever kind of finbox you install must be laminated to the deck of your board. Without it, a board manufactured from ESP will not have the necessary reinforcement to maintain the integrity of the finbox over time. You can augment A Boxes to work, but it's probably a lot less work to with PB or Tuttle instead.

a twin fin would need about 1.33 more fin area than the single fin, you could install the boxes and use 'about' a pair of 45cm to achieve the 'equal to about' 601cm of area the single 65cm fin has. so 2 45cm to replace the long pointer.

or install the surf type fins as a 2+1 set up. which needs between 1.33 and 1.5 more fin area, with this you could use side fins around 12cm and existing center 'about' 38--43cm, as the Makani Hahalua weed fin, and have a significant area, but being a weed fin, no such a long long depth

no certain I would hasard a guess how this Starboard would react, but I had a similar issue and did this to a Tabou Rocket 135L, the ProBox side fin boxes are plenty strong enough for smaller fins up to around 15.5cm,
this setup does most things better than a single fin, except top speed, slightly slower and slower to plane, both caused by increased drag, the up sides are great up wind and turns like a rat with Nikes, plus short depth in shallow or rocky waters

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